1. Field of the Invention
Embodiments of the invention relate to a method of manufacturing a silicon carbide semiconductor device and a silicon carbide semiconductor device in which silicon carbide (SiC) is used as a semiconductor material and in particular, in which current flows from a front surface side to a rear surface side of a semiconductor substrate.
2. Description of the Related Art
A silicon carbide semiconductor material has a large bandgap compared to a silicon (Si) semiconductor material and therefore, has high dielectric breakdown field strength. On-resistance, which is resistance in a conduction state, is inversely proportional to the third power of the dielectric breakdown field strength and therefore, for example, with widely used silicon carbide semiconductor materials called 4-H type (4-layer periodic hexagonal crystal:4H-SiC), the on-resistance may be suppressed to 1 few-hundredths of that of a silicon semiconductor material.
Therefore, combined with their property of high thermal conductivity enabling easy heat dissipation, silicon carbide semiconductor materials are expected for use in next generation power semiconductor elements having low loss. For example, silicon carbide semiconductor elements of various structures such as Schottky barrier diodes, metal oxide field effect transistors (MOSFETs), PN diodes, insulated gate bipolar transistors (IGBTs), gate turnoff thyristors (GTOs), and the like that use a silicon carbide semiconductor material are being developed.
A material and method of forming an ohmic electrode, important in forming a silicon carbide semiconductor element, have been presented (for example, refer to Japanese Patent Application Laid-Open Publication No. H1-268121). It has been shown that in an n-type region, when nickel (Ni) is used as a material and heat treatment is performed at about 1000 degrees C. under reduced pressure or in an inert gas atmosphere, a nickel silicide is formed and this silicide functions as an ohmic electrode.
On the other hand, an ohmic electrode in a MOSFET is formed in an ohmic contact hole opened in an interlayer insulating film, after a gate oxide film, a gate electrode, and the interlayer insulating film are formed. It has been shown that by annealing in a case of forming such an ohmic electrode, an unexpected reaction progresses at a MOS interface and since the MOS interface property is subject to severe damage, the annealing temperature has to be suppressed to 850 degrees C. or less (for example, refer to Japanese Patent Application Laid-Open Publication No. 2003-243654).
A technique of controlling annealing to 850 degrees C. or less when the ohmic electrode is formed has been disclosed (for example, refer to Japanese Patent Application Laid-Open Publication No. H7-99169). In this technique, after a nickel silicon alloy is formed, heat treatment at 700 degrees C. or less is performed whereby a silicide is formed by a solid-phase reaction of the nickel silicon alloy, without reacting the silicon carbide semiconductor substrate and the nickel silicon alloy. Further, a technique of using laser irradiation after nickel silicon alloy formation to locally perform heat treatment for short periods and suppress the amount of carbon that reaches the ohmic electrode surface to a concentration that is less than that of the nickel element has been disclosed (for example, refer to Japanese Patent Application Laid-Open Publication No. 2012-99598).
Further, it has been disclosed that when a nickel silicide is formed on a silicon substrate, by defining the nickel film thickness and annealing temperature, the composition of the nickel silicide to be formed may be controlled. It has been shown that this is because a solid-phase reaction at a contact interface of the nickel and silicon begins from about 250 degrees C. and progresses dependent on the temperature whereby the composition of nickel silicide formed is uniquely determined by the nickel film thickness.